Surfaces of metal and ceramic substrates have been modified in a variety of ways to improve and alter their characteristics. Unfortunately, various shortcomings have been experienced, depending upon the modifying treatment employed.
For example, surfaces have been modified using a plasma/flame process that shoots hot metal particles towards a surface. Upon impact, the particles are mechanically bonded to the surface. This process is performed in the open air which results in oxidation of the metal particles. Oxidation is undesirable because it prevents metallurgical bonding.
Surfaces have also been modified using mechanical processes such as abrasion, shot peening and scribing. Typically, these mechanical processes result in modification on a macroscopic scale and therefore have little effect on a microscopic scale. Furthermore, scribing cannot be done on a ceramic substrate and provides only a limited number of modification patterns.
Chemical etching and milling modify the surface by adding or removing molecules. The chemicals utilized frequently are harmful to the environment and therefore their use is undesirable.
Surfaces have been modified by placing a polymer layer on a surface and then embedding particles in or on the polymer layer. A typical problem encountered with this process is the encasement of particles by the polymer which reduces the effectiveness of the particles and the surface modification. Also, the polymer may delaminate thus removing the surface modification.
Another method for coating surfaces with hard substances requires heating a powder on the surface to be modified to 900.degree. to 1200.degree. C. See, for example, U.S. Pat. No. 4,749,594 to Malikowski et. al. Many surfaces cannot withstand these extremely high temperatures without undergoing deformation, degradation or other undesirable change.
Apart from the matter of surface modification as mentioned above, in a typical prior art brazing operation a flux is used to clean (deoxidize) and protect the metal surfaces to be joined and to promote flow of a brazing alloy. For aluminum alloys, the brazing temperature is typically in the range of about 570.degree. to about 610.degree. C., i.e., about 1,050.degree. to about 1,130.degree. F., depending on the flux and brazing alloy used. The relatively high brazing temperatures result in a number of drawbacks. Metal alloys having a melting temperature less than 570.degree. C. cannot be used. Substrates that would experience deformation or chemical change when exposed to these temperatures cannot be used. A brazing oven in which the brazing is performed must be set at this high temperature.
Residual flux from the prior art brazing operation must often be removed because it can be corrosive in the presence of water and it can adversely affect paint/glue adhesion (the paint/glue adheres satisfactorily to the flux but the flux adheres poorly to metal). Thus, a step is required to remove the flux. The more flux that is used the higher the cost for the total amount of flux and the more residual flux that must be removed. Conventional removal steps, e.g., washing and thermal cracking, are not only inconvenient, but are often not effective in removing all of the residual flux or flux reaction products..
A paste including a powder brazing alloy and flux can be utilized in place of a braze clad metal to which flux is applied. However, the same problems with respect to residual flux exist. Additionally, the paste can be dislodged in the normal course of handling the surface prior to heating, leaving sections of the surface without the brazing alloy and flux, thereby creating the potential for a poor or nonexistent braze joint.